This invention relates to an improvement in electrical cables, especially electrical cables of the type utilized for transmission of high frequency signals, such as radio and television signals. Cables of this type have one or more inner conductors, usually formed of copper or copper-clad aluminum, with the inner conductor or conductors being surrounded by a dielectric material, such as expanded or unexpanded polyethylene, polypropylene or polystyrene. The dielectric material, in turn, is surrounded by a metallic outer conductor, typically a metal tube or metallic foil. The metallic outer conductor, in addition to serving as an electrical conductor, also serves to shield the cable against leakage of RF radiation and to seal the cable against ingress of contaminants, such as moisture, which may undesirably affect the electrical characteristics of the dielectric or cause corrosion of the conducting surfaces of the inner and/or outer conductor.
In this regard, it is well recognized that corrosion of the metallic conductors is highly deleterious to the RF propagation characteristics of the cable. At the high frequencies normally used in such cables, due to the phenomenon known as the "skin effect", the R.F. electrical signals are conducted entirely along the conducting surfaces or skin of the inner and outer conductors. Any disruption of these surfaces, such as would occur due to corrosion or pitting of the conductor, increases the signal loss of the cable. This condition of corrosion, should it occur at the interface between the conductor and a terminating connector, can further contribute to the intermodulation of R.F. carriers being transmitted along a coaxial cable in a FDM (frequency division multiplex) system. This is particularly true at higher frequencies (greater than 300 MHz) where the entirety of the RF power would otherwise reside within the dimensions of the contaminating oxide. Furthermore, where the inner and outer conductor contributes significantly to the mechanical and physical behavior of the cable, the presence of corrosion will deteriorate the strength and fatigue behavior of the cable construction. This deterioration may permit the ingress of contaminating moisture into the dielectric resulting in disrupted transmission, the ingress of ambient RF energy into the coax, or more seriously the egress of transmitted RF power which is capable of interfering with ambient communication, navigational or control signals.
In an effort to protect the conductors against corrosion, a common prior practice in the commercial production of coaxial cables has been to provide a protective adhesive coating on the conductor which forms a physical barrier to water or water vapor. Typically the adhesive employed is a copolymer of ethylene with an ethylenically unsaturated carboxylic acid, usually acrylic acid, as is disclosed for example in prior U.S. Pat. Nos. 3,272,912; 3,681,515; and 4,107,354. The adhesive coating, in addition to excluding moisture and protecting the conductor surface against corrosion, also serves to adhesively bond the conductor to the adjacent dielectric material to contribute to the physical properties of the cable. The use of ethylene acrylic acid (EAA) adhesives in this manner is widely practiced in the industry.
In spite of the widespread commercial use of this adhesive, certain significant problems are presented thereby. Although the EAA adheres quite tenaciously to the inner conductor, it is not totally effective in preventing moisture from coming into contact with the conductor. Thus, the EAA coating still allows some corrosion of the inner conductor, particularly at the ends of the cable. This is especially true in situations where the concentrations of EAA have been diluted to promote the removal of the adhesive coating. Additionally, since the adhesive can bond so strongly to the inner conductor, it is difficult, during installation, to strip the adhesive from the conductor when installing connectors. Unless the adhesive is properly stripped from the conductor, a poor connection can occur. This poor connection can be manifested by intermittent open circuits, inability to transmit higher power levels, selective performance with frequency, or the nonlinear transport of signals resulting in intermodulation. Further the EAA adhesive, being a polar molecule, produces increased electrical dissipation in the cable at the high frequencies of the signals carried by the cable. In recognition of this, in commercial practice, attempts are made to minimize the dissipation or loss contributed by the EAA adhesive by applying the minimum possible thickness of EAA adhesive to the center conductor. However, even at these minimal levels, the effect of the EAA adhesive is measurable.